Vehicles may be fitted with evaporative emission control systems to reduce the release of fuel vapors to the atmosphere. For example, vaporized hydrocarbons (HCs) from a fuel tank may be stored in a fuel vapor canister packed with an adsorbent which adsorbs and stores the vapors. At a later time, when the engine is in operation, the evaporative emission control system allows the vapors to be purged into the engine intake manifold for use as fuel. However, leaks in the emissions control system can inadvertently allow fuel vapors to escape to the atmosphere. Thus, various approaches are used to identify such leaks.
One example approach for leak detection is shown by Annoura et al. in U.S. Pat. No. 7,107,759. Therein, a brake booster vacuum pump is used during engine-off conditions to apply a vacuum on the fuel system. Fuel system leaks are then identified based on a rate of decay of the applied vacuum. This allows the same vacuum pump to be used for brake booster application as well as leak detection.
However, the inventors herein have identified potential issues with such an approach. As one example, to perform the leak detection routine, the vacuum pump has to be operated, consuming vehicle power and reducing fuel economy. As another example, some leaks may be masked in the presence of negative pressure. If the leak goes undetected, exhaust emissions may be degraded.
Thus, in one example, some of the above issues may be addressed by a method for a vehicle fuel system, comprising: indicating fuel system degradation in response to a change in fuel system pressure following application of a positive pressure generated at an electrically-driven vacuum pump. In this way, air exhausted from a vacuum pump during pump operation can be applied to the fuel system for leak detection.
In one example, an engine system may include a vacuum pump configured to supply vacuum to a vacuum consumption device (e.g., a vehicle brake booster). The vacuum pump may be coupled to a fuel system such that during conditions when the vacuum pump is operated to supply vacuum to the vacuum consumption device, air exhausted from the pump can be directed to the fuel system. For example, the exhausted air can be applied on a fuel system canister to pressurize a fuel tank. Following application of the positive pressure, a rate of pressure decay may be monitored. In response to the rate of pressure decay being higher than a threshold rate, a fuel system leak may be determined. The vacuum pump may also be operated to apply a vacuum on the fuel system so that each of a positive pressure and a negative pressure leak test can be sequentially performed. Following application of the negative pressure, a rate of vacuum decay may be monitored and a fuel system leak may be determined if the rate of vacuum decay is higher than a threshold rate.
In this way, vacuum pump operation can be synergistically coupled to a positive pressure fuel system leak test. By allowing positive pressure from the vacuum pump to be used for a fuel system leak test while a vacuum pump is already operating to provide vacuum to a vacuum-operated actuator, the electrically driven vacuum pump may be operated less frequently. By reducing the frequency of pump operation, an operational lifespan of the electrically driven vacuum pump may be increased. In addition, since fuel vapor does not pass through the pump, issues related to material incompatibility may not arise. By using the vacuum pump to also perform a negative pressure leak test, leaks masked by a positive pressure leak test may be identified during the negative pressure leak test, and vice versa, improving leak detection accuracy. By using the same pump for each of a positive pressure and negative pressure leak test, as well as for vacuum generation for other engine actuators, component reduction benefits are achieved.
It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description, which follows. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.